Ablation catheters are well recognized and important tools for conveying an electrical stimulus to selected locations within the human body. Ablation catheters have been used for many years for the treatment of certain types of cardiac arrhythmia. For example, ablation catheters have been used to interrupt or modify existing conduction pathways associated with arrhythmias within the heart. Ablation procedures are also used for the treatment of atrial ventricular (AV) nodal reentrant tachycardia. Accepted treatments of this condition include ablation of the fast or slow AV nodal pathways. Known cardiac ablation procedures focus on the formation of lesions within the chambers of the heart at selected locations which will either prevent the passage of electrical signals associated with atrial premature contractions or prevent the formation of improper electrical pathways within the heart which can result in atrial arrhythmia.
Radio frequency (RF) catheter ablation has become increasingly popular for many symptomatic arrhythmias such as AV nodal reentrant tachycardia, AV reciprocating tachycardia, idiopathic ventricular tachycardia, and primary atrial tachycardias. Nath, S., et al., "Basic Aspects Of Radio Frequency Catheter Ablation," J Cardiovasc Electrophysiol, Vol. 5, pgs. 863-876, October 1994. RF ablation is also a common technique for treating disorders of the endometrium and other body tissues including the brain.
A typical RF ablation system in its most basic form comprises an RF generator which feeds current to a catheter containing a conductive tip electrode for contacting targeted tissue. The system is completed by a return path to the RF generator, provided through the patient and a large conductive plate, which is in contact with the patient's back.
The standard RF generator used in catheter ablation produces an unmodulated sine wave alternating current at frequencies of approximately 500 to 1000 kHz. The RF energy is typically delivered into the patient between the conductive tip electrode of the catheter and the large conductive plate in contact with the patient's back. During the delivery of the RF energy, alternating electrical current traverses from the conductive tip through the intervening tissue to the back plate. The passage of current through the tissue results in electromagnetic heating. Heating tissue to temperatures above 50.degree. C. is required to cause irreversible myocardial tissue injury. However, heating tissue to temperatures above approximately 100.degree. C. at the electrode/tissue interface can result in boiling of plasma and adherence of denatured plasma proteins to the ablation electrode. The formation of this coagulum on the catheter tip causes a rapid rise in electrical impedance and a fall in the thermal conductivity, resulting in loss of effective myocardial heating. Nath, S., et al., "Basic Aspects Of Radio Frequency Catheter Ablation," J Cardiovasc Electrophysiol, Vol. 5, pgs. 863-876, October 1994. Moreover, such extreme heating of the tissues can damage healthy tissue surrounding the targeted lesion.
Because of the dangers of overheating tissue with ablation catheters, systems for controlling the temperature at the ablation site are necessary. Such systems have been in use for many years. Common ablation systems for controlling the temperature at the ablation site contain an electrode as well as a thermocouple or thermistor at the tip of the catheter. In these systems, a pair of wires from the thermocouple extend back through the body of the catheter to an amplifier in an electrical control portion of the system. An output from the amplifier, is indicative of the temperature of the heated tissue and is used by a control unit to control the duty cycle or power level of the RF generator. This arrangement permits regulating the amount of RF energy delivered to the tissue to control the temperature at the target tissue. An example of a system in which the duty cycle of the ablation catheter is controlled by a temperature sensor is disclosed in U.S. Pat. No. 5,122,137 entitled "Temperature Controlled RF Coagulation."
Known RF ablation systems that use temperature control mechanisms have numerous disadvantages. First, additional wires are required for the connection to the thermocouple. Each additional wire is a reliability and manufacturing problem when constructed in a long, thin catheter. Second, the transmission of a low voltage signal from the thermocouple to the amplifier, which is indicative of the temperature, must be transmitted accurately over a long distance in order to appropriately limit the temperature. Maintaining an accurate transmission is very difficult because the low voltage signals from the thermocouple are being transmitted by wires directly adjacent the wire used to provide the high voltage signal for ablating. The low voltage signals from the thermocouple are typically swamped by the high voltages and high frequencies used for the ablation, thereby causing temperature signals to be very noisy and less likely to give accurate temperature readings. Finally, in the event of an electronics fault, there is no mechanism in the known devices for current limiting or fusing capability to protect the patient and/or catheter.